1. Field of the Invention
The present invention relates to an objective lens for reading information on an optical disc (disc having information recorded at high density), and more particularly, to an objective lens for reading information on a compact disc as whose light source semiconductor laser is used.
2. Description of the Prior Art
An objective lens for reading information on an optical disc needs high resolving power in order to detect very small information recorded on the optical disc at high density. In addition, the objective lens itself need be small in size and light in weight, since the lens is caused to be moved in order to effect automatic condensing function and tracking function. Also it is necessary to lengthen working distance as long as possible in order not to produce contact between the optical disc and the objective lens. Even if there are incomplete light source and imperfect optical axis of the objective lens, and errors in arranging the optical axis of the objective lens with respect to the optical disc surface, good correction of aberration with respect to angle of view more than a predetermined angle is needed in order to obtain good signal information.
On the one hand, in order that a great deal of compact disc players are supplied as consumer appliance the objective lens which is cheap and keeps stable in function is required. However, since in the objective lens of this kind each lens component bears excessive burden of aberration, affection of tolerance in actual producing is great, so that good signal in use is not obtained often, and excessive adjustment is forced for increasing producing performance of the lens so that an inexpensive supply of the lens can not be effected.
An objective lens of two-component three-lens structure has been known by, for example, U.S. Pat. No. 4,270,843 (corresponding to the Japanese laid-open patent application No. 4068/1980). This objective lens has a numerical aperture (N.A.) in the range of 0.45 to 0.5 and is small in size and light in weight. However, since somewhat irregularity in aberration balance is, remarkably high degree of accuracy is needed in stages of production and assembling. Accordingly, this objective lens is insufficient still to keep quality of the lens stable and high in mass production.